A “well log” comprises data collected along the path of a hole in the ground. Such holes are referred to as “wells” in the art of oil and gas exploration. A well log “curve” is a sequential collection of one category of data, such as resistivity or gamma ray activity. A well may have several logs, and a log may have several curves.
A primary task in the search for oil and gas is to gain an understanding of the distribution and nature of rocks and fluids in the subsurface. This understanding is important for the success and the efficiency of the search. Well logs provide direct information about what is in the subsurface. Data collected by logging wells can have significant economic consequences because wells may cost millions of dollars and oil deposits can be worth billions of dollars.
Several logs are commonly acquired from each well. Data are usually acquired by lowering sensing tools into the hole by cable. The cable holds the tools and maintains electrical connection with recording equipment at the surface. Data are acquired by the sensors and “logged” (recorded) at the surface as the tool is pulled up the hole. Data may also be acquired by instrumentation at the bottom of the hole while drilling is in progress. Data collected in or descriptive of the rock and fluid surrounding the hole fall into the category of “well log data”.
Well logs provide detailed and direct measurements of rock and fluid properties in the subsurface. Examples of such measurements are (a) gamma ray intensity, which relates to the types of minerals present; (b) electrical resistance, which relates to the quantity and types of fluids; and (c) sonic velocity (the time required for sound to travel from sender to receiver), which relates to both rock and fluid properties. These three examples are illustrative of the hundreds of well logs that may be collected.
Unfortunately, gaps are sometimes present in well logs. Well log gaps result in less information on which to base a model and more uncertainty regarding what will be encountered when the next well is drilled. Gaps are present for a variety of reasons. Tools may fail or malfunction, or operators may turn off recording equipment at the wrong time. It may be discovered after the fact that the wrong interval was logged.